Why does a plane shake in a clear sky, and is turbulence dangerous?

A plane shakes in a clear sky because the air itself is moving unevenly, even when there is no cloud to show it. Clear-air turbulence is the turbulent movement of air masses with no visual clues, and it is caused when bodies of air moving at widely different speeds meet. Where one stream rubs past another, wind shear can produce vortices, and the air tends to move chaotically.

At the heights airliners fly, the region most prone to it is the high troposphere, around 7,000 to 12,000 metres, and it is most often found near jet streams. The trouble is that you cannot see it. It is not possible to detect with the naked eye and very difficult to detect with a conventional radar, so pilots often cannot anticipate it. Thin cirrus clouds can sometimes hint that it is likely.

Is it dangerous? The honest answer is mostly a matter of comfort and occasionally of safety. Clear-air turbulence can be hazardous to the comfort, and occasionally the safety, of air travelers, because a sudden encounter can impart significant stress to the airframe. That is why seats are designed to withstand strong forces during turbulence, and why seat belts are designed to keep passengers in their seats.

The oldest seat belt story is a human one. In 1910 the pilot Benjamin Foulois used a simple strap so he could remain at the controls during turbulence. More than a century later, the advice is the same: when the sign is on, buckle up.